The present invention relates to an arrangement for transmission and reception of electromagnetic radiation, consisting of a transmitting and receiving part for transmission and reception of the electromagnetic radiation, and a reflecting part, arranged so as to reflect the transmitted electromagnetic radiation back to the transmitting and receiving part, while passing through a medium which transmits radiation. The transmitting and receiving part includes an electromagnetic radiation source and a concave main reflector which is common to transmission and reception.
Transmission and reception of electromagnetic radiation is of interest in a number of applications, for example in measurement of air pollution, distance measurement etc. In this connection, use is made of visible or invisible light, for example, which is transmitted over a measuring distance, after which the light is collected and analyzed. In order to be able to transmit light over great distances without too great losses in intensity, use is made of telescopes. In this connection, a light source is positioned close to the focus of a parabolic or spherical mirror. After reflection in the mirror, light in the form of a parallel luminous beam is transmitted. After transmission through the atmosphere, a part of the luminous beam is collected with a receiving telescope constructed in similar manner and is focussed into a spectrometer, either by direct connection or via an optical fibre. In the case of direct connection, use is made, for example, of a so-called Newton arrangement with a mirror arranged at an angle in front of the receiving mirror.
An alternative to positioning transmitter and receiver at either end of the measuring distance is to transmit the radiation out and back over the measuring distance via a reflector. In this connection, all the components in the system must be accurately aligned, since transmitter and receiver are normally positioned next to one another, so that the radiation which is reflected back must be directed in a different direction from the transmitted radiation.
A suitable type of reflector for positioning at the far end of the measuring distance would be a so-called retroreflector, which is an optical component in which a number of reflecting areas are arranged in such a manner that the light which strikes the same is returned in the opposite direction, even in the case of incident light which deviates to a certain extent from the direction of the normal. By virtue of the fact that a retroreflector usually has a relatively high angular tolerance, the requirement for alignment and stability is relatively low. However, use of retroreflector in the known arrangement described above, with transmitter and receiver positioned side by side, would still occasion problems. The maximum intensity of the reflected light would in fact go back into the transmitting telescope and only a small part would reach the receiver.